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horseshoe crab

horseshoe crab, large, primitive marine arthropod of the family Limulidae, related to the spider and scorpion and sometimes called a king crab (a name also used for the largest of the edible true crabs). The heavy dark brown exoskeleton, or carapace, is domed and shaped like a horseshoe. The body is divided into a broad, flattened, semicircular front part (the prosoma), a tapering middle part (the opisthosoma), and a pointed, spiky taillike part (the telson).

Horseshoe crabs have no jaws, and the mouth is flanked by a pair of pincerlike chelicera that are used to crush worms and other invertebrates taken as food. Five pairs of walking legs attached to the prosoma enable the animals to swim awkwardly or burrow through the sand or mud. The respiratory organs are called book gills and are unique to horseshoe crabs. Each book gill is made of about 100 thin leaves, or plates; these are fitted like pages of a book onto one pair of flaplike appendages on the opisthosoma. Rhythmic movement of the appendages circulates water over the gill surfaces and drives blood into and out of the gill leaves.

Horseshoe crabs first appeared in the Upper Silurian period, and a number of fossil species have been described. Four species still survive; three of these are found along the Pacific coast of Asia. The American species, Limulus polyphemus, is common along the Atlantic coast from Nova Scotia to Florida. It lives in shallow water, preferring soft or sandy bottoms, and reaches a maximum length of nearly 2 ft (61 cm). The shores of the Delaware Bay form the largest spawning ground of the species, and their eggs make the bay a critical feeding stopover for migrating shorebirds.

Horseshoe crabs are considered living fossils; they resemble fossil trilobites and eurypterids of the Paleozoic era. They are classified in the phylum Arthropoda, subphylum Chelicerata, class Merostomata, order Xiphosura.

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Merostomata

Merostomata (phylum Arthropoda) Class which includes the king crabs (horse-shoe crabs) and the extinct eurypterids or ‘water scorpions’. Eurypterids were usually 10–20 cm in length, but some were much larger, e.g. Pterygotus which grew up to 2 m long. They ranged from the Ordovician to the end of the Palaeozoic. The horseshoe crabs also appeared in the Lower Palaeozoic and have survived to the present.

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Merostomata

Merostomata (phylum Arthropoda, subphylum Crustacea) Class that includes the horseshoe crabs (king crabs) and the extinct eurypterids. The eurypterids (‘water scorpions’), which grew up to 3 m long, ranged from the Ordovician to the end of the Palaeozoic; the horseshoe crabs also appeared in the Lower Palaeozoic and have survived to the present.

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horseshoe crab

horse·shoe crab • n. a large marine arthropod (class Merostomata) with a domed horseshoe-shaped shell, a long tail-spine, and ten legs, little changed since the Devonian period.

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horseshoe crab

horseshoe crab See MEROSTOMATA.

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Merostomata

Merostomata See Chelicerata.

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